doc: NODE_EXTRA_CA_CERTS is ignored if setuid root

Fixes: https://github.com/nodejs/node/issues/22081

PR-URL: https://github.com/nodejs/node/pull/23770


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>